A light emitting diode (LED) is composed of an epitaxial structure such as a homo-structure, a single hetero-structure, a double hetero-structure, or a multiple quantum well. The LED with a p-n junction that can emit light with various wavelengths has several beneficial characteristics, including low electrical power consumption, low heat generation, long lifetime, small volume, good impact resistance, fast response and excellent stability. These characteristics have made the LED to be a light source applied into electrical appliances and electronic devices.
Typically, an LED is composed of an epitaxial structure with a substrate, an n-type cladding layer formed on the substrate, a p-type cladding layer and an active layer formed between the n-type cladding layer and the p-type cladding layer. Light is emitted as current flows through the active layer. The light wavelength can be altered by varying the composition of the epitaxial structure material.
In general, the light output of an LED depends on the quantum efficiency of the active layer and the light extraction efficiency. The higher the quantum efficiency of the active layer, the higher the light output of the light-emitting diode. Generally, improving the quality of epitaxial structure and the structural design of the active layer increases the quantum efficiency of the active layer. In addition, as the light extraction efficiency increases, the light output of the light-emitting diode is enhanced. In order to improve the light extraction efficiency, efforts are made to overcome the significant photon loss resulting from total reflection inside the light-emitting diode after emission from the active layer.
Currently, a common method for increasing the light output of light-emitting diode devices is to increase the light extraction efficiency of the light-emitting diodes. The methods for increasing the light extraction efficiency of light-emitting diodes substantially includes several methods described as following. The first method is to roughen the surface of the light-emitting diode by directly etching the surface of the light-emitting diode, so as to achieve the effect of increasing the light extraction efficiency of the light-emitting diode. In the method of roughening the surface of the light-emitting diode, a mask protects the local region of the surface, and wet etching or dry etching is applied to roughen the surface. However, in this conventional method for roughening the surface, the uniformity of the roughened surface is poor. The second method is to change the shape of the light-emitting diode by etching. However, the process of the second method is more complicated, so that the process yield is low.